In a conventional gas generator of the above type, a gas generating agent containing sodium azide as a base has been used. Since harmful slags and mist are generated due to the use of sodium azide, it has been proposed to use a nonazide gas generating agent in which a nitrogen containing compound other than inorganic azide is combined with an oxidizer. However, the nonazide gas generating agent generally has a linear burning velocity of 30 mm/sec or less under a pressure of 70 kg/cm.sup.2, and it is difficult to burn this type of gas generating agent with high stability to achieve stable or constant output performance. An example of a gas generator for an air bag using a nonazide gas generating agent for achieving stable combustion has been proposed in International Publication No. WO 96/10494, wherein a rupture plate is provided at an opening that allows passage of a gas generated by the gas generating agent, and the maximum pressure inside the gas generator is controlled by adjusting the strength and thickness of the rupture plate and the size of the opening. When the rupture plate is used in an attempt to control the internal pressure of the gas generator in this manner, however, it was found that the start of actuation or operation of the gas generator tends to be delayed since the rupture plate does not rupture unless a pressure greater than a certain level is applied to the plate, burning of the nonazide gas generating agent is not stabilized, which often results in variations in the output performance of the gas generator, and the amount of CO gas generated as a result of the combustion is increased.